Organic electroluminescent devices, which may be referred to as organic EL devices, having an organic emitting layer have been promisingly expected to be used as large-area full-color display devices of a solid-state emitting type, and many developments thereof have been performed.
Recently, practical use of organic EL device displays has been started, and thus the organic EL devices have been desired to have a higher luminance, a higher efficiency and a longer lifetime.
A technique of incorporating a very small amount of a fluorescent molecule (dopant) into an organic emitting layer made mainly of a host material is known in order to meet the above-mentioned desires.
There is also disclosed, for example, an organic EL device having an emitting layer including a mixture of a first component for accepting positive charge carriers (holes) from a first-charge-carrier injecting layer (hole-transporting layer), a second component for accepting negative carriers (electrons) from a second-charge-carrier injecting layer (electron-transporting layer), and a third component which is an organic emitting component for emitting light by recombination of the charge carriers from the first and second components wherein at least one of the first, second and third components is combined with the others of the first, second and third components, so as to form a semiconductor interface of type II (see, for example, Japanese Patent Application National Publication No. 2002-507825).
In this device, the carriers (holes and electrons) injected in the emitting layer are made into a state that their electric charges are separated, whereby the device gives highly efficient luminescence.
Besides, the following are disclosed: an organic EL device having two components of an emitting material and a dopant for improving the dispersibility thereof (see, for example, Pamphlet of WO 02/104080); an organic EL device made of a host material capable of maintaining both of injection and recombination of holes and electrons, a first dopant capable of accepting recombination energy in the host material, and a second dopant capable of trapping holes from the host material (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-038140); an organic EL device including a host compound and a single aromatic polycyclic hydrocarbon compound and a fluorescent dye (see, for example, JP-A No. 2000-106277); an organic EL device wherein two or more condensed aromatic dopants are used (see, for example, Japanese Patent No. 3509746); an organic EL device having a first dopant and a second dopant which gives a longer wavelength than the first dopant (see, for example, JP-A No. 9-134786); a blue-light emitting organic EL device including at least two dopants, at least one of which is smaller in affinity level than a host (see, for example, JP-A No. 2004-171828); and an organic EL device including a blue dopant and a warm color dopant wherein the amount of the warm color dopant is such a level that light is not emitted therefrom (see, for example, JP-A No. 2004-221045).
As described above, the techniques of incorporating a dopant into an organic emitting layer are very important for improving the emission efficiency of an organic EL device and improving the emission lifetime thereof. Thus, various improvements have been made.
However, it cannot be said that organic EL devices having a sufficient efficiency and lifetime have been developed.
In light of the above-mentioned problems, an object of the invention is to provide an organic electroluminescent device having a particularly long lifetime.